Illustrated herein in various embodiments are electrophotographic imaging members and more specifically, processes for preparing imaging members by forming a dispersion of a charge generating material in a polymer matrix in a solvent system using n-butyl acetate and methyl isobutyl ketone.
In an electrophotographic application such as xerography, a charge retentive surface (i.e., photoconductor, photoreceptor, or imaging surface) is electrostatically charged and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as “toner.” Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface.
The aforementioned process is known, and useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be image-wise discharged in a variety of ways. Ion projection devices where a charge is image-wise deposited on a charge retentive substrate operate similarly.
The electrophotographic imaging members may be in the form of plates, drums or flexible belts. These electrophotographic members are usually multilayered photoreceptors that comprise a substrate, a conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer, an optional overcoating layer and, in some belt embodiments, an anticurl backing layer.
A conventional technique for coating cylindrical or drum shaped photoreceptor substrates involves dipping the substrates in coating baths. The bath used for preparing photoconducting layers is prepared by dispersing photoconductive pigment particles in a solvent solution of a film-forming polymer. However, the choice of pigment particle, polymer, and solvent solution is critical in achieving a high-quality photoconducting layer.
In this regard, some organic photoconductive pigment particles cannot be applied by dip coating to form high quality photoconductive coatings. For example, phthalocyanine pigment particles tend to settle, which necessitates constant stirring to ensure a uniform dispersion. However, stirring can lead to entrapment of air bubbles that are carried over into the final photoconductive coating deposited on a photoreceptor substrate. These bubbles cause defects in final prints due to differences in discharge of the electrically charged photoreceptor between the regions where the bubbles are present and where the bubbles are not present. Thus, for example, the final print will show white areas over the bubbles during discharged area development or dark spots when utilizing charged area development.
Moreover, many pigment particles tend to agglomerate when attempts are made to disperse the pigments in solvent solutions of film-forming polymers. This agglomeration leads to non-uniform photoconductive coatings which in turns lead to other print defects in the final xerographic prints due to non-uniform discharge. These defects can be seen in streaking and charge-deficient spots. The film-forming polymer selected for photoconductive pigment particles in a charge generating layer can adversely affect the particle dispersion uniformity, coating composition rheology, residual voltage after erase and electrophotographic sensitivity. Some polymers can lead to unstable pigment particle dispersions which are unsuitable for dip coating photoreceptors. Thus, for example, when a copolymer reaction product of 86 weight percent vinyl chloride and 14 weight percent vinyl acetate such as VYHH terpolymer from Union Carbide is utilized to disperse hydroxygallium phthalocyanine (OHGaPc) photoconductive particles, an unstable dispersion is obtained. Additionally, a charge generating layer containing this copolymer has poor light sensitivity and gives high residual voltage after erase.
Furthermore, combinations of some polymers can result in unacceptable coating or electrical properties. For example, some polymers are incompatible with each other and cannot form coatings in which the polymers or particles are distributed uniformly throughout the final coating. Similarly, the choice of solvent affects the quality of the dispersion and the ease of the manufacturing process. For example, a polycarbonate binder, poly(4,4′-diphenyl-1,1′-cyclohexane carbonate), dissolved in tetrahydrofuran or toluene results in a non-Newtonian dispersion.
Along this line, these issues are also disclosed in Nealey et al., U.S. Pat. No. 6,017,666; Nealey et al., U.S. Pat. No. 5,681,678; Nealey et al., U.S. Pat. No. 5,725,985; Burt et al., U.S. Pat. No. 5,456,998; and Nealey et al., U.S. Pat. No. 5,418,107, the disclosures of which are totally incorporated herein by reference. While these patents propose the production of a charge generation layer matrix using binders and solvents to enhance photoconductive particle dispersion uniformity, etc., further improvements are still desired.
Thus, there is a need for additional processes and compositions to form a charge generating layer of an imaging member that exhibits enhanced dispersion stability, enhanced charge transport, etc.